I don’t know if this is what you sister in law meant, but I have heard this used to actually divide up the Bible into St. Paul’s writing verses other scriptures. The claim is that since St. Paul was sent by God to preach to the gentiles, gentiles should (they say) rely mostly on his writing to the gentiles, (thus also dismissing his letter to the Hebrews.)
The person I spoke with this about believes that Paul preached a different gospel than the other apostles, “the gospel of grace”, (as if Matthew, Mark, Luke and John don’t contain grace.) The words of Jesus are interpretted based on their interpretation of St. Paul’s writings. If there is any apparent contradiction, such as Jesus reminding us what we do for others we do for Him, that is dismissed as being told to the Jews. (She won’t actually claim that she “dismisses” the other writings, but that is basically what she does.)
The person I know who told me this seems to me to be making an extreme effort to continue her believe in salvation by faith alone. There are numerous examples in the Gospels and letters such as James that refute that claim by discussing the role of works. But in saying those books where intended for the Jews, she doesn’t need to reconcile the whole Bible, because she divided it up into parts she adheres to and parts she doesn’t.
Ironically, if you read the surrounding verses in the second chapter of the Second Letter to Timothy, you’ll see find the words “rightly dividing” used in this way takes that phrase grossly out of context.
